Velocity modulation tubes



May 24, 1960 A. LERBS VELOCITY MODULATION TUBES 15 Sheets-Sheet 1 Filed May 5. 1955 A. LERBS VELOCITY MODULATION TUBES May 24, 1960 15 Sheets-Sheet 2 Filed May 5. 1955 May 24, .1960

A. LERBS VELOCITY MODULATION TUBES Filed May 5, 1955 15 Sheets-Sheet 5 May 24, 1960 A. LERBS VELOCITY MODULATION TUBES l5 Sheets-Sheet 4 Filed May 5, 1955 A. LERBS VELOCITY MODULATION TUBES May 24, 1960 15 Sheets-Sheet 5 Filed May 5, 1955 May 24,, 1960 15 Sheets-Sheet 6 Filed May 5, 1955 a a. M E m. w 9 H n /M 2 U K w k m s u 2 d H r L J -i 9 A. N k F 2L1 a z w i a b w. Fifi J1 v 3 i A. LERBS VELOCITY MODULATION TUBES May 24', 1960 15 Sheets-Sheet 7 Filed May 5, 1955 May 24., 1960 A. LERBS VELOCITY MODULATION TUBES 15 Sheets-Sheet 8 Filed Ma 5, 1955 May 24, 1960 A. LERBS 2,938,139

VELOCITY MODULATION TUBES Filed May 5, 1955 15 Sheets-Sheet 9 y 24., 1950 A. LERBS .VELOCITY MODULATION TUBES l5 Sheets-Sheet 10 Filed May 5, 1955 FIG. 19

A. LERBS VELOCITY MODULATION TUBES May 24, 1960 15 Sheets-Sheet 11 Filed May 5, 1955 FIG. 20

FIG.24

4, 1960 A. LERBS 2,938,139

VELOCITY MODULATION TUBES Filed May' 5, 1955 1 l5 Sheets-Sheet 12 1 FIG. 21

A. LERBS VELOCITY MODULATION TUBES May 24, 1960 15 Sheets-Sheet 13 Filed May 5, 1955 Pic-5.27

24, 0 A. LERBS 2,938,139

VELOCITY MODULATION TUBES Filed May 5, 1955 15 Sheets-Sheet 1 4 M I 0 1 I \i. F '4 May 24, 1960 A. LERBS VELOCITY MODULATION TUBES l5 Sheets-Sheet 15 Filed May 5, 1955 where this density is weaker.

, order of the wavelength to be produced.

United States Patent 2,938,139 VELOCITY MODULATIONTUBES Alfred Lerbs, Paris, France, assignor to Compagnie Gen- The present invention relates to velocity modulation electron discharge tubes.

In known tubes of this type, the velocity of the electron beam is modulated in a first circuit whereby the electrons become bunched. This bunching produces a high-frequency field in a second circuit to which. the electrons give up their energy. This conversion of velocity modulation into density modulation occurs along the beam; the density of the electrons is uniform in each transverse section of the beam; such a density modulation is termed longitudinal modulation. 2 The operation of tubes based on this principle and con structed for millimeter wavelengths is not very satisfactory from the point of view of efficiency and useful power. One of the main reasons for this resides in the necessity of giving the beam and in consequence the cathode negligible'transverse dimensions relative to the wavelength,

thus making it impossible to use intense beam currents.

Further, the dimensions of the various elements of the tube become so small that it is ditiicult to obtain sufiicient precision in construction. 7 I

The present invention has for its object to provide tubes which are adapted to operate within the aforementioned frequency range and are exempt from these disadvantages, in'that they enable the use of cathodes whose transverse dimensions are of the same magnitude order as the wavelength.

In the tubes embodying the invention, the electron beam is velocity modulated by an ultra-high frequency field in a modulating space. At the end of a certain path, the conversion of velocity modulation into density modulationis effected. According to the invention, this modulation in density is transversal with respect to electron beam. In other words, each transverse section of the beam comprises areas where the electron density is stronger than the mean density of the beam and areas 0n the other hand, the mean electron density of successive transverse sections issubstantially the same.

The tubes according to the invention comprise another space, said collecting space, in which the transverse density modulation of the beam induces another ultra-high frequency field; the dimensions of the cathode, and of both modulating and collecting spaces, are of the magnitude The invention will be better understood with reference to the accompanying figures in which:

Fig. 1 is a simplified view of significantelements of the tube embodying the invention seen in longitudinal section,

1 the operation of which is explained with reference to the diagrams shown in Figs. 2 2 3 and 4;

Figs. 5 and 6 are respectively views of an amplifier seen in longitudinal and transverse sections, embodying the invention;

Fig. 7 is a perspective view of a detail of the tube of Figures 5 and 6; V v 7 Figs. -8 and 9 are respectively plane and sectional views of an oscillator embodying the invention;

Patented May 24, 1960 detail of the tube of Fig. 9;

Figs. 11 to 13 are longitudinal views of embodiments of oscillators according to the invention with adjusting means for the oscillation frequency; V.

Fig. 14 is a view in longitudinal section of an embodiment of a frequency multiplier;

Fig. 15 is a view in longitudinal section of an amplifier according to the invention in which the modulating ultrahigh frequency field is travelling; i

Figs. 16 and 17 are views in longitudinal section of subsequent improvements in the circuits utilized;

Figs. 18 and 19 are similar views of two amplifiers having non-rectilinear circuits;

Fig. 20 is a cross sectional view of an oscillator of circular form;

Figs. 21 and 22 are views of two subsequent enrbodiments of amplifiers having-non-rectilinear:circuits;

Figs. 23 and 24 are similar views of two subsequent embodiments of oscillators of circular for-m;

Figs. 25 to 29 are side elevation views. of. five variants of magnets suitable in. the tubes according to the -'invention.

Figs. 30 and 31 are axial and transverse sectional views, respectively, of an oscillator of circular form operating with a non-uniform magnetic field due to thecirculation of current in an axial rod.

In Fig. 1, a flat emitting cathode 1 is seen in longitudinal section. Disposed in front of this cathode and parallel thereto are two flat grids 3 and 4. Two other fiat and parallel grids 7 and 6 are respectively disposed in the planes of the grids 4 and 3. A uniform magnetic induction field B is oriented in a direction perpendicular to the plane of the figure. These four electrodes are brought to a positive potential V relativeto the cathode 1. An ultra-highfrequency field is set. up by any suit- I high frequency field being induced in the space 5. by

the transversely modulated beam.

In the ensuing description, for the sake of simplification, it will be supposed that the electrodes, 3 and 4 are at such distance from the cathode 1 that the efi'cct the field B had on the electrons in the space between the electrodes 1 and 4 may be neglected and that, in other words, the electrons emitted by the cathodel, traverse the space 2 in a direction perpendicular to the grids 3 and 4. I

In Figure 1 the cathode is seen in longitudinal section and its length l is substantially equal to the wavelength in the free space of the ultra-high frequency field set up velocity of all the electrons at the output endofvthe 1 modulating space 2 is governed by thepotential V, but some of these electrons are retarded and others accelerated by the modulating field in accordance with the well known phenomenon of velocity modulation by an ultra-high frequency field.

The electron trajectories or paths are thereafter in curved in the drift space between the electrodes and 7--6 as a result of the. action of the field B' and assume the form of arcs of circles. After having travelled along-a semi-circle whose radius is proportional to their initial velocity at the outlet end of the space 2, the electrons attain the space between the electrodes 6 and 7. Now, it is well known that the time taken by the electrons in travelling along a semi-circle in a space in which a uniform magnetic field prevails, is constant and is independent of their initial velocity, in this space, the angular velocity along this semi-circle, depending only on the intensity of the field; the initial velocity has determined the radius of said semicircle.

' Thus the electrons emitted at the same instant t by the cathode 1 reach the space 5 at the same time. Assuming that a, b, c, d and e are the semi-circles travelled through by the electrons emitted respectively by the cathode in front of the nodes A, C, E and of the antinodes B, D, of the ultra-high frequency field on the cathode, it is obvious that the radius of the circles a, c and e is constant with respect to time, the action of the high-frequency field having no efiect on the corresponding electrons.

If the configuration of the ultra-high frequency field along the cathode is as seen in Fig. 2,, at a time t (maximum positive amplitude at the antinode B, and maximum negative amplitude at the antinode D) the circle b would have maximum radius, the electron emitted at B having been subjected to the greatest acceleration of the part of the ultra-high frequency field. Similarly, the circle d would have minimum radius, the electrons emitted at D having been subjected to maximum deceleration by the ultra-high frequency field. T being the period of variation of the ultra-high frequency field, at the time the configuration of the field is as plotted Fig. 2

It is obvious that in the course of one total period of variation of the field, the circles a, c and e maintain a constant radius, the circles b and d having a radius which varies in accordance with a sinusoidal time law from a maximum to a minimum.

If at the time t, the field has the configuration shown in Fig. 2,, the semi-circle b has a maximum radius, the semi-circle d minimum radius, and the ends of the corresponding trajectories B, D are displaced nearer to the end C of the semi-circle c.

It is clear, therefore, that in the vicinity of point C there isa region of maximum electron density at time t+0, 0 being the constant time taken by the electrons in travelling from the space 2 to the space 5.

- Inthe foregoing description, it was implied that the ultra-high frequency field was nil at the nodes, or, in other words, that the field was perfectly standing. It is clear that if this hypothesis is not actually fully realized, the phenomena would be qualitatively the same.

Figs. 3 and 4 show the network of electron trajectories at the two successive instants t and the density is uniform, the ultra-high frequency modulating field being overall nil, and having no action over the. electrons.

Therefore it can be understood that the density has a maximum variation during a period of the modulating ultra-high frequency field in the vicinity of A, C and E.

Thus there is established in the space 5 by means of induction, another ultra-high frequency field termed a collectingfield which is standing and of which the respective antinodes are A, C and E.

A first embodiment of an amplifying tube of the invention is shown in longitudinal sectional and cross sectional views, respectively, in Figs. 5 and 6. In these figures the same reference numerals designate the same elements shown in Fig. 1. a

The tube comprises in an evacuated metal envelope 14 a cathode 1 having a filament 9ufed. by conductors 10. An electron lens 8 focuses the electronic emission in a direction perpendicular to the cathode. The space 2 and its two grids 3 and 4, hereinafter termed the modulating circuit, are disposed just about the cathode. The space 5, where the amplified energy is received, is disposed. in such. manner that, the grids 5 and 6 are respectively in the planes of the grids 3 and 4. These two circuits are separated by the electrode 31. A collector 18 captures the electrons. A source of direct current voltage 33 has its negative terminal connected to earth and, furthermore, to the unit 2, 5, 14, 18 and 31.

The envelope 14 is disposed between two polar pieces 32 (Fig. 6) which create a magnetic field B in'the envelope which is perpendicular to the plane of the Fig. 5. A water jacket 15 comprising an inlet 16 and an outlet 17 serves to cool the unit.

A section of a wave guide 11 coupled to an ultrahigh frequency source not shown, terminates in space 2. Like- .wise, the circuit 5 is coupled to an output 12. Windows 13 render the envelope vacuum tight. v

Fig. 7 shows, in perspective, the circuit 2. The latter has the shape of a prism made of metal comprising a reentrant portion. This reentrant portion is closed, by the electrodes 3 and 4 in the form of grids. The circuit 5 not shown has an identical shape.

The operation of this unit is obvious from the foregoing description. The wavelength of the operating frequency is governed by the longitudinal dimension of the circuit 2. The length of the electrode 31 is seiected in accordance with the field B and the applied potentials, in such a manner that all the electrons crossing the space 2 impinge the collector 18, after passing through space 5. These factors determine the radii of the circles a, c and e by well known methods.

The circuit 2 is closed by a short-circuit (31). A standing electromagnetic field is established therein. If the length of the circuit is appropriate, this field will oscillate and a field node will be established at the two ends. I

Likewise, in the circuit 5,. a node will be established at the end 31. But it is clear from the foregoing description that if the circuit 2 vibrates with the nodes facing the two ends of the cathodes, the circuit 5 vibrates with the antinodes situated at the limits of the impacts of the electrons.

Figs. 8 and 9 show in longitudinal sectional and horizontal cross-sectional views, respectively, a self-oscillating tube embodying the invention. The circuits 2 and 5 are two cavities shown respectively in cross-section and plan in Fig. 10. Like reference numerals designate like elements in Figs. 5 and 7. A coupling between the cavities 2 and 5 is established by means of a coaxial line 19 which terminates at each end in a coupling loop 20. This coupling creates a feedback between the spaces 2 and 5. The tube comprises a single ultra-high frequency output 12 in the form of a guide. The frequency of the oscillations may be adjusted by means-of slightly deforming the circuits 2 and 5 by conventional means.

.Figs. 1,1,to 13 diagrammatically illustrate some oscillators embodyingthe invention. r

- to frequency multiplication.

In g- 11-1; t e c cuitsfl and. .-.i h. h are in he-f m. of. two portions of a rectangular; wave-guide having the same cross-section, are conneetedby'a wave guide22;-.having a smaller cross-section. This guide 22 couples the spaces 2 and5 to eachother and creates the feedback necessary for. sustaining oscillations. In practice, the guide 22 is obtained by means of a block 23 disposed between the portions of guides 2 and 5. The length and the height of this block governs the phase and amplitude of the feedback. The energy is obtained at the output 12 of the guide 25.

In Fig. 12, there is also provided, in derivation with the guides 2 and: 5, a guide portion 34 which emerges from the'tube; A'window 13 disposed in wave portion 34-, renders the-tubevacuum tight. A-piston-disposedin the part of the guide 34 outside the tube allows the adjustment; of the feedback.

The tube shown in Fig. 13. is similar to;.that shown in Fig. 11 but; the position of the block 23 isadjustable by means of, a mechanical system comprising a rod. 24 which is slidable inside a bellows 25. In tightening, the nuts 35, pressure is brought to bear on the plate 36 and the rod 2'4, rigid with the block 23, is displaced which permits adjustmentzof'the feedback. a 1 The tubes embodying the invention are also applicable Fig. 14 shows merely by .way of example afrequency doubler. It differs from the amplifier shown in Fig. 5 only inrespect of. the circuit 5 which, instead of permitting a free passage with respect to the electrons over its entire-surface, comprises apertures 26 permitting such passage-incertain regions while preventing it in other regions. The lengthof this space is-equal to; 1.25%; x being the wavelength. of the operating frequency. The apertures 26 are disposed at distances from the left. end. of the circuit 5: which; are respectively equal to r x s, x s+i 2 ami+x The left end of the circuit 5-is short-circuited and an ultra-high frequency node is established. Three antinodes are established in front of the apertures 26. Thus a standing ultra-high frequency field is established as indicated at, the base of the figure. 1

Itwill now be supposed that the modulating ultra-high frequency field instead of being standing is travelling. Referring to Fig. 1, it can be seen that at the instant t, the field is distributed along the cathode in the manner shown in Fig. 2,. At this instant, the density bunching is obtained in the same manner as in the case of the standing field. At instant t+At,; this field pattern is displaced towards the right, for example, of a, length AS which'isequal to Mt, v being the velocity of the travelling field. In the collector circuit the maxima: ofthe field are displaced in the samemanner .andat the same velocity v as the maxima of the modulating field. Thus a travelling field is established oncondition that the collector circuit is closed on its wave impedance and that, furthermore, the field is capable of being propagated at the velocity v. Hence it is necessary that the ultra-high frequency parameters of the two circuits be identical.

Fig. 15 shows an amplifier in which the modulating field is travelling. This tube on the whole comprises substantially the same elements, as the tube shown in Fig. 5 which carries. the same reference characters. It comprises. a vverylong guide 27. to which'a travelling wave is fed .at- -11,1 alongv this guide there being disposed several identical cathodes 1 which, are separated by a distance which is a whole number multiple of Ac, he bein'gfthe'wavelength in the guide of the travelling field. Further, the field'B is so selected that the electrons emitted-from one cathode strike the homologous point of the following cathode, in the absence of the ultra-high frequency field. I V v As it travels} the wave velocity modhlates-the beam I the same beam which is density modulated transversely.

at the end of its semi-circular trajectory. The amplified wave therefore gives to the beam of the second cathode a deeper velocity modulation. Finally, there is received at 12 a Wave which has undergone several successive amplifications. In order to reduce the risk of self-oscillations, a localized attenuation 28 is disposed in the guide 27' between two cathodes 1. The respective dimensions of the circuits are so selected as to avoid' harmful elec-' tron impacts.

In Fig. 16, plates 37 have been disposed in such manner as to receive the electrons that have traversedthe induced ultra-high frequency field. The source of voltage 38 brings them to the same direct current potential less than that of the circuit 5. They are preferably disposed in front of the voltage nodes inthe circuit 5. The electrons that traverse the circuit 5 in the vicinity of these nodes have not given up muchenergy tothe ultra-high frequency field and have notin consequence lost kinetic energy. Thus, it is advantageousto retard them so as to lose as little energy as possible at the moment of their capture.

It is possible to design tubes embodying the invention with cathodes and circuits having, in the directio-nperpendicular to the plane of: the Figs. 1, 5 and 8, a dimension of the order of the operational wavelength. Fig. 17 shows a sectional view, taken on a plane parallel to the lines of force of the magnetic field, i.e. on a plane perpendicular to the plane of Fig. 5, of a space or circuit 2 employed in these tubes. There may be seen in this figureseveral systems of electrodes 3-4, excitation also being possible in a standing high-frequency wave mode. The grids 3-4 are disposed in front of the antinodes of the exciting wave, since no advantage is gained if the electrons pass through the regions of the nodes where little energy is given upor received by them. The circuit 5 (not shown) is identical to the circuit'2.

In the foregoing, cathode 1, electrodes 3 and 4 were supposed to be at suchv distance that the jtimetaken by the electrons in transit was constant and independent in particular of their initial velocity in the space without electric field and electromagnetic field.

It is, however, difficult to construct tubes, having cathode, and accelerating electrodes, i.e., grids 3 and 4 in very close proximity. The potential V'can be of the order of 1000 v. relating to the cathode and breakdowns can occur.

In the ensuing description, the case in which the cathode 1 and the electrodes 3 and 4 are sufficiently spaced apart for avoiding such breakdowns, and located in such manner that it is no longer possible to neglect the action of the magnetic field B on the electrons travelling 'between the electrodes 1 and 4, is examined.

Electrons emitted by the cathode lare: in'thiscase subjected in the space 2 to the joint action of the highfrequency field and the field B and enter the drift space, between electrodes 3-4 and 67, the angles between the respective trajectories and electrodes 3 depending on the phase of the modulating field at the instant of emission and of the emission point of the cathode. Actually, the direction of electron trajectories is 'no more normal to electrodes 3 and 4 and parallel to the lines of force of the ultra-high frequency field. 'The trajectories in this space are no longer semi-circles, the arcs travelled along depending on the phase of the high-frequency field, and the transit time is no longer constant. It may be shown that the transit time varies, between an. upper and a lower limit, about a mean value i.e. is'modulated. Theory and experience show that, for the tube accord It may also be demonstrated that forcertain difference 7 values between the respective transit times of the electron, the resulting bunching is such that the tube operates substantially as described above. These values may be determined by experiment.

To this end electrodes 6 and 7 of the collecting circuit may be disposed as shown in Fig. 18 i.e. in planes parallel to the planes of the electrodes of the modulating circuit 2, but vertically raised away from the plane of the cathode '1.

As shown in Fig. 19, it is also possible to give the guide 11-12 an incurved form with the convexity directed towards the cathode 1. Compared with the tube shown in Fig. 5, such tubes have the advantage of requiring weaker magnetic fields, for the same mean transit time of the electrons.

In the tubes shown in Figs. 18 and 19, the trajectories 41 are clearly shorter than a semi-circumference and the electrons in travelling along these trajectories within a given mean transit time require a weaker magnetic field than in the respective arcs were semi-circles. For the same transit time, angular velocity is lower when the angular trajectory is shorter and it is well known that the angular velocity is proportional to the magnetic deflecting field.

Fig. 20 shows an oscillator derived from the amplifier shown in Fig. 19. The guide 27 is in the form of a circle, each cathode 1 being situated at the apex of a square disposed outside this circle. The structure of the system is symmetrical about an axis perpendicular to the plane of the figure. A guide 12 serves as an ultra-high frequency output. However, in the present case, the guide 27 forming a complete loop, a standing ultra-high frequency field will be induced more easily than a travelling field. A short circuit 29 fixes the position of a voltage node in the guide and hence the position of the nodes and anti-nodes of the standing field produced in this guide and, in consequence, the positions of the cathodes and of the grids. This oscillator permits the magnet necessary for its operation to be smaller.

Instead of making the electron trajectories smaller than half a circle, it is also possible, as shown in Figs. 21, 22 and 23, to keep these trajectories substantially equal for all the electrons, in spite of the fact that, as mentioned above, the respective angles of incidence of these trajectories on the electrodes 2 differ from each other.

The tubes shown in Figs. 21 and 22 are respectively analogous to those in Figs. 18 and 19, but in Fig. 21, the circuit 5 is lowered with respect to the circuit 2 instead of being raised. In Fig. 22, the guide 1112 is incurved, its concave side facing the cathode.

In Fig. 23, the cathodes are placed at the corners of a square within the circle 27.

Owing to the vertically-staggered arrangement of the circuits 2 and 5, the electron trajectories--if the electron were to enter the drift space with a direction perpendicular to the electrode 3would be longer than semi-circles.

This would result, neglecting the distance between the electrodes '1 and 4, in a modulation of the transit time.

It may be shown that this modulation of the transit time may be compensated by the effect of modulation of the transit time due to the distance between the electrodes, for a convenient value of the accelerating field V and the deviating field B. In other words, if the values V and B are suitably chosen, the electron trajectories are as shown in Figs. 21 through 23 again semi-circles the latter being longer normal to the electrodes 3 and 4. The modulation of the transit time is thus eliminated; the modulation of the beam is therefore efiected in conditions similar to those prevailing in the tube shown in Fig. 5.

Fig. 24 shows an oscillator which is similar to that shown in Fig. 20, but which comprises an additional cyldrical electrode 42 insulated from the other electrodes of the tube.

This electrode is brought to an adjustable potential .which is somewhat higher than that of the electrode 27,

by means of the potentiometer 43 and a source 44, whose terminals are respectively connected to the ends of the potentiometer 43. Thus there is obtained in the drift space 42-27 an electrostatic radial field whose lines of force are oriented, according to the setting of this potentiometer, in the direction 27-42 or 4227. If they are in the direction 42-27 (the potential of the electrode 42 being greater than that of the guide 27) the electron trajectories 41 are no longer arcs of circles so that the electrons approach the electrode 42. The transit time is therefore increased.

Theory and practice show that, by suitably adjusting the potentiometer 43, the transit times of the electrons may be made independent of their initial velocity in the space and operation is as though the trajectories were semi-circles. 7

Similarly, if the direction of the lines of force is 27- 42, the electron trajectories recede from the electrode 42 and are shortened. The transit times are reduced and-by means of a suitable adjustement-it is possible to obtain the optimum values of the transit time indicated above.

Similar results may be obtained by imparting to the lines of force of the magnetic field in the drift space a suitable pattern instead of using a uniform field as in the foregoing examples.

To this end, polar elements 32 may be used which are convex (Fig. 25) or concave (Fig. 26) or annular (Fig. 27), the result obtained with annular elements being the same as that obtained with the elements shown in Fig. 24. All these elements may be used with a cylindrical tube 14 such as for example those shown in Figs. 20, 23 or 24. The axis of the cylinder is indicated at A, A. In the case of tubes having a rectilinear'structure, the polar elements may have an incurved shape such as those in Fig. 28 or 29, which shows the system in the same way as Fig. 6. The cathode is assumed located at the base of the figure. The distance between the polar elements increases and the magnetic field decreases when approaching the cathode in Fig. 28 and receding from the cathode in Fig. 29.

If it is desired to reduce the mean transit time of the electrons, for example so as to bring it to its optimum value, it is necessary to subject the electrons in the drift space to a magnetic field which is the stronger as the electrons are further from the cathode. The effect of such a magnetic field is to accentuate the mean curvature of and hence shorten the trajectories.

Conversely, if it is so desired to lengthen the electron trajectories so as to compensate various factors affecting the transit time and hence render them independent of the initial velocity of the electrons in the drift space, there is created in the drift space a field which is such that the electrons are subjected to a field whose intensity decreases with recession of the electrons from the cathode. The etfect of this field is to decrease the mean curvature of and hence lengthen the trajectories.

In the first case, the magnets shown in Fig. 25 may be used with the tubes shown in Fig. 20 or 24, or the magnets shown in Figs. 26 or 27 used with the tube shown in Fig. 23. The magnets shown in Fig. 28 would be used with the tube shown in Fig. 5.

In the second case, the polar elements shown in Fig.

25 are used with the tube shown in Fig. 23 and the elements shown in Figs. 26 and 27 with the tubes shown in Fig. 20 or 24 and those shown in Fig. 29 with the tubes shown in Fig. 19.

Figs. 30 and 31 are axial sectional and cross-sectional views respectively of another embodiment of an oscillator tube in which the electrostatic and magnetic fields in the drift space are non-uniform, the two above-mentioned effects being combined.

The metal cylindrical envelope 14 contains, axially disposed therein, and insulated therefrom, a metal rod 42.- The latter is connected between theterniinals-uf a source of low voltagealternating energy 44 and, furthermore, to a tap of potentiometer 43 by means ofwhich it is possible to bring the rod to a positive or-negative potential relative totheenvelope 14.- The'curren't created in the rodbythe. source 44 setsfup around the latter an. alternating magneticfield whose lines offorce are coaxial with the tube, the intensity of the field decreasing with recession from the rod. The positive, of negative, potential appliedttothe rod results=in the creation in the drift space of a, radial electric. field similar to that shown in Fig. 24.

Further, the cathodes 1 are disposed at the apices of a regular polygon which is coaxial with the tube and is disposed in the median plane of the latter. Guides 27 are disposed on a cylinder coaxial with the envelope 14 and extend in a direction parallel to the generatrices of this cylinder. They are connected to the envelope 14 so as to be brought to the same potential as the latter. Each guide is disposed opposite the corresponding cathode. These guides are disposed side by side and coupled together. by apertures 45. One of the guides is provided with an output 12 the passage through the envelope being closed by an insulating window 13. Each guide comprises a modulating circuit 2 and two collector circuits 5, 5 disposed on either side of the modulating circuit; these circuits are, for example of the type shown in Fig. 7. The electrons from the cathodes 1 pass through the circuits 2 and, under the action of the nonuniform alternating magnetic field, describe trajectories attaining the collector circuit 5 or the circuit 5, according to the alternations. The shape of these trajectories is also affected by the adjustable radial electric field; the cathodes 1 are unblocked by the pulses of the negative potential relative to the potentials of the guides 27. These pulses are applied by a source 46 which is synchronized with the source 44 by means known per se and generally indicated by the dot-dash line 47. The source 46 produces a pulse for each positive or negative peak of the voltage furnished by the source 44. The electrons therefore follow alternately the trajectories 41 or 41". In both cases, each guide is the centre of an ultra-high frequency field, as was explained in the course of the description with reference to the Figs. 5 to 7.

' Owing to coupling by means of the apertures 45, the

ultra-high frequency fields of all the guides are in phase and the energy may be provided through a single output 12.

What I claim is: 1. A microwave electron tube comprising within an "evacuated envelope at least one pair of microwave circuits comprising a first and a second circuit extending in tablishing within said envelope a magnetic field having lines of force perpendicular to said direction and to said beam for directing the electrons of said beam along curved trajectories whereby, due to combined action of longitudinal modulation of said first circuit and of said magnetic field, the longitudinal modulation of said beam is transformed into transverse modulation thereof, asecond microwave circuit extending across said trajectories in coupled relationship with said beam, said second circuit being of sufficient length and so disposed as to be acted upon by different densities of the electron beam transversely of the latter andalong said second circuit,

2. A tube according to claim 1, wherein-saidsecnd microwave circuit extends in prolongation of said first circuit. r q

3. A tube according. to claim 1, wherein said second microwave circuitis offset with respect td said firstcircuitt 4. A tube according to claim 1, whereiri s'aid first and said second circuits areparallel.

5. A- tube according to claim I wherein said first and said second circuits are portions of a curved wave guide.

6. A tube as claimed in claim 1, wherein said means for establishing said magnetic field provide a field which is non-uniform through the area traversed by said trajectories.

7. A tube as claimed in claim 1, further comprising collector means associated with said second circuit.

8. A tube as claimed in claim 1, wherein said first and second circuits extend in said predetermined direction, and in a second direction normal to said direction.

9. A tube as claimed in claim 1, further comprising reflecting means for generating standing waves in said first circuit.

10. A microwave electron tube according to claim 1 wherein at least one of said microwave circuits is formed by parallel grids extending transversely of the beam.

11. A microwave electron tube according to claim 1 wherein coupling means is provided at each of said microwave circuits to provide coupling externally of the tube.

12. A microwave electron tube according to claim 1 wherein the shape and location of said microwave circuits relative each other provides trajectories of different lengths for electrons at different transverse parts of the beam.

13. A microwave electron tube structure according to claim 1 wherein said microwave circuits are connected to form a structure presenting a generally convex surface facing the path of said electron beam.

14. A microwave electron tube structure according to claim 1 wherein said microwave circuits are connected to form a structure presenting a generally concave surface facing the path of said electron beam.

15. A microwave electron tube according to claim 1 wherein said second microwave circuit is offset relative said first circuit to increase the length of said trajectories.

16. A microwave electron tube according to claim 1 wherein said second microwave circuit is olfset relative said first circuit to decrease the length of said trajectories.

17. A microwave electron tube comprising within an evacuated envelope means for emitting in a first direction an electron beam having a cross section elongated in a second direction generally normal to said first direction, circuit means including a first microwave circuit having means extending in said second direction for passing said beam transversely therethrough in coupled relationship with microwave energy along said circuit, means for feeding microwave energy .to said first circuit to provide amplitude differences of an ultra high frequency field at different points along said first circuit for simultaneously velocity modulating electrons in said beam differently at said different points in accordance with the amplitude of the high frequency field at the respective point-s, means for establishing within said envelope a magnetic field having lines of force perpendicular to said directions for directing the electrons of said beam along curved trajectories, said circuit means including a second microwave circuit extending across said trajectories in coupled relationship with said beam, said magnetic field establishing means acting on the differently modulated electrons passing different points of said first microwave circuit to determine the trajectories of said electrons such that said electrons are modulated and bunched in a direction trans- 

